1. Field of the Invention
The present invention relates generally to a semiconductor memory device, and more particularly, to a quantum-well memory device (QWMD) and a method for making such a device.
2. Description of the Related Art
Presently, nanoelectronic technology has been utilized to fabricate quantum-well memory devices (QWMD). Such a device, using quantum-well structure in its floating gate, can be scaled well below a 50 nm gate length. Self-limited processes determine the dimensions of quantum-wells in a QWMD. Besides excellent charge retention, a QWMD also has the advantages of fast programming and erasing.
FIG. 1A shows the fabrication process of one known quantum-well memory device 100. As shown in FIG. 1A(1), a gate oxide (GOX) 120 is grown on top of a p-type substrate 110, and a polysilicon gate 130 is deposited on top of the GOX 120. FIG. 1A(2) shows that the GOX 120 is then etched with diluted hydrofluoric (HF) acid during a self-limited wet etching process, resulting in undercuts 120′. Next, a layer of bottom oxide (BOX) 140 (formed on the substrate 110 surface) and a conformal layer of top oxide (TOX) 150 (formed over the polysilicon gate 130) are defined during an oxidation process. FIG. 1A(3) indicates that a conformal layer of polysilicon 160 substantially fills the undercuts 120′ and covers the BOX 140 and the TOX 150. As shown in FIG. 1A(4), an oxidized layer 170 is formed by oxidizing the polysilicon 160 until the outer encapsulating portion of the polysilicon 160 is converted into an oxidized layer except for the portion of polysilicon 160 embedded at the undercuts 120′. As a result, the un-oxidized portion of polysilicon 160 creates polysilicon inserts 180. Finally, two junctions 190a and 190b are implanted next to the poly inserts 180.
One drawback of the device 100 is the simultaneous growth of the TOX 150 and the BOX 140 during the oxidation process. Another drawback of the device 100 is the structure deformation after the oxidation process, which is shown in FIG. 1B. In addition, the diluted HF acid used in the wet etching process results in the undesired non self-limited etching on the shallow trench isolation (STI) structure along the channel width, as is shown in FIG. 1C.
In view of the foregoing, there is a need for a new quantum-well fabrication method and device that can not only provide fast programming and erasing performance and good data retention characteristics, but also overcome the above-mentioned drawbacks of the known QWMD 100.